1. Field of the Invention
This invention relates to audio signal transmission systems and, more particularly, to the detection of noise in frequency modulation transmission systems.
2. Description of the Prior Art
It is often desirable to have available an indication of the noise content of a signal received through a particular transmission system. In many systems where the transmission medium is relatively stable, a long-term approximation of the noise can be made by transmitting test signals of predetermined characteristics through the system and measuring the signal-to-noise ratio directly. In other types of transmission systems, the transmission medium is relatively unstable and the amount of noise which may be present in received signals can vary over a wide range. If the transmission medium is subjected to varying ambient conditions, for example, the noisiness of the medium may change significantly over time and present a recurring necessity for determining the noise level.
In yet other forms of transmission systems, the transmission medium changes so rapidly and over such extremes that a constant running indication of noise level is necessary for the proper operation of the system. Such systems include long-range radio transmission systems subject to fading and line-of-sight microwave and light frequency wave transmission systems, particularly when the transmitter and receiver are movable with respect to one another. When such systems are used for the transmission of voice signals, it has been found desirable to provide a squelch circuit which is responsive to the noise level in the system and which heavily attenuates the high-level noise signals.
In a hands-free telephone station system in which it is contemplated that the transmitter be portable and carried about with the user as the user moves about the room, the transmission path varies considerably during normal usage. Moreover, if transmission frequencies in the light wave frequency range are utilized to improve the privacy of such a system, the line-of-sight transmission path does indeed vary considerably. In such systems, instantaneous and continuous noise detection and suppression are essential.